Method of producing a black oxide coating on ferrous metals



United States Patent 3,279,957 METHOD OF PRODUCING A BLACK OXIDE COATING ON FERROUS METALS Richard R. Fink, Waukesha, Wis., assignor to D. J. Campbell Co., Inc., Waukesha, Wis., a. corporation of Wisconsin No Drawing. Filed Dec. 17, 1962, Ser. No. 244,919

5 Claims. (Cl. 148--6.14)

This invention relates to a process of treating ferrous metals and more particularly to a process for providing a black oxide coating on steel surfaces.

Black oxide coatings are applied to ferrous metals such as steel to prevent serious rusting under storage conditions. 'In the conventional process, the steel article is immersed in an aqueous alkaline bath containing oxidizing agents. The bath is heated to a temperature of about 300 F. and the steel article is maintained in the heated bath for a period of about 5 to 20 minutes. After this period of immersion, the article is removed, cooled and Washed and the resulting coating on the steel is black in color and is generally considered to be ferroso-ferricoxide.

In the conventional black oxide process, it is necessary to maintain precise ratios between the water and the salts in the treating bath. At the elevated temperature of about 300 F., the Water will boil and vaporize with the result that a portion of the water is continually lost which changes the weight relationship between the water and the salts. To maintain this ratio, it is necessary in the conventional process, to add water during the operation, and the norm-a1 process requires special temperature controls in the form of temperature sensors which are operably connected to a water valve and serve to continuously add water to the bath.

The addition of water to a solution at an elevated temperature in the range of 300 F. can be dangerous and explosive. The tank must be specially built to introdue the water in such a way as to prevent sudden formation of large quantities of steam. This, again, increases the cost of the tank construction.

As the conventional process uses an open tank, vapors are given and special ventilating equipment is required.

The present invention is directed to an improved process for providing a black oxide coating on ferrous metals. According to the invention, the treating solution containing a specific water-salt mixture, is contained within a closed, hermetically sealed pressure vessel. The solution is heated to a temperature in the range of 285 to 308 F. and a pressure of 57.5 to 75.4 p.s.i.a. At this temperature, the water will boil and vaporize forming steam in the upper part of the closed vessel, but the water will not be lost to the process.

It has been found that a specific relationship of the salt composition to water is necessary when using a closed vessel due to the fact that water is present not only in the solution, but also is present as steam or vapor in the upper portion of the vessel.

The present method produces a uniform black oxide coating and eliminates discoloration between areas. In some cases, with the conventional methods of black oxide treatment, a green or red cast may appear in certain areas of the article.

The process can be easily controlled for no water is added during the process. With the proper addition of heat, the temperature is maintained within the prescribed range. This process eliminates the expensive temperature sensing equipment which is needed in the conventional process in which water is introduced into the tank and also simplifies the tank construction itself. This results 3,279,957 Patented Oct. 18, 1966 in a substantially reduced equipment cost, even though the process requires a pressure vessel as opposed to the open tank used in the conventional process.

The vessel to be employed in the process of the invention can be any usual small pressure vessel which can be hermetically sealed and which Will have the required strength to withstand pressures generally not exceeding 100 p.s.i.a. The vessel should contain a conventional thermometer or temperature indicating device so that the temperature of the treating solution can be noted.

The treating solution is a conventional black oxide solution which is basically an alkaline aqueous solution containing one or more oxidizing agents. A salt mixture to be added to the water to form the treating solution generally contains from 50 to 90% by weight of an alkaline metal hydroxide, such as sodium hydroxide or potassium hydroxide, and the balance being the oxidizing agents. The oxidizing agents may be alkali metal nitrates, nitrites, chlorates, permanganates, peroxides, hy posulfites and the like. The alkali metal is generally potassium or sodium. A salt composition that can be used in forming the treating solution is as follows in parts by weight:

Alkali metal hydroxide 50-90 Alkali metal nitrite 3-20 Alkali metal nitrate l-50 The particular composition of the treating bath is not critical and there are numerous salt compositions sold on the market which can be employed with the process of the invent-ion. Several of these are Blackmax X-5, sold by MacDer-mid, Inc., of Waterbury, Connecticut, .Ebonal-S and Ebonal-SBO, sold by Enthone, Inc., of New Haven, Connecticut, and Blakodize, sold by Chemical Coroporation, Springfield, Massachusetts.

To provide the desired black oxide coating when using the closed pressure vessel, it has been found that a specific. water-salt ratio should exist. More specifically, in forming the treating solution, one part of water should be used to .77 to 1.31 parts of the salt composition. When the treating solution is heated to an elevated temperature, the water will boil and vaporize with the result that a portion of the water will be maintained as vapor or steam in the upper portion of the vessel. To provide for this water in the form of vapor, as well as the water in the treating solution, the above-mentioned ratio between salt and water is required.

In carrying out the process, the ferrous or steel article is immersed in the treating solution, the vessel is hermetically sealed, and the solution is then heated to a temperature in the range of 285 to 308 F., and preferably in the range of 285 to 290 F., by any convenient means. For example, the vessel can be heated by a gas flame which can be manually applied, or a gas burner can be operably connected to a thermostat or temperature sensing device located within the vessel.

As the vessel is hermetically sealed, the temperature will determine the pressure within the vessel and at the abovementioned temperature range, the pressure will generally be in the range of 52.7 to .4 p.s.i.a.

The steel article is maintained within the heated treati-ng solution for a period of 10 to 25 minutes and after this period, the heat is removed, the vessel is opened and the article is removed and rinsed with water. The coating produced by this process has a deep black color and is uniform throughout all areas of the article.

A specific illustration of the process of the invention is as follows:

14.2 ozs. by weight of a salt composition was dissolved in 12.1 ozs. of water in a pressure vessel having an inside diameter of 5 inches and a height of 6 inches. The salt mixture had the following composition in weight percent:

Sodium hydroxide 74.0 Sodium nitrate 17.0 Sodium nitrite 8.0 Sodium hyposulfite 1.0

The vessel was hermetically sealed and heat was applied to the vessel by a gas burner to heat the treating solution to a temperature of 290 F. which result in a pressure of 57.5 p.s.i.a. The steel article was immersed in the treating solution for a period of 10 minutes after which the vessel was opened and the steel article was removed and rinsed in water. The coating on the steel article was a uniform deep black color.

The present invention has decided advantages over the conventional process using an open tank. The process of the invention uses a fixed amount of Water and it is not necessary to add additional amounts of water during the treating operation. This results in a simplification of equipment in that water conduits, valves and temperature sensing devices are not required.

In addition, as the tank is hermetically sealed, foreign materials cannot contaminate the bath which may cause discoloration of certain areas of the article.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointed out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. A method of producing black oxide coatings on ferrous articles, comprising the steps of introducing a treating solution into a vessel, said treating solution comprising a salt composition dissolved in water with the Water being present in a ratio of 1 part water to .77 to 1.31 parts of said salt composition and said salt composition comprising from 50 to 90% by Weight of an alkali metal hydroxide and the balance being at least one strongly oxidizing agent, immersing the ferrous article in the solution, hermetically sealing the vessel, heating the solution to a temperature of 285 to 308 F. while maintaining the hermetically sealed condition in said vessel for a period of 10 to 25 minutes, and thereafter removing the article from the treating solution.

2. A method of forming a black oxide coating on ferrous articles, comprising the steps of dissolving .77 to 1.31 parts by weight of a salt composition in 1 part of water to form an oxidizing solution, said salt composition containing 50 to 90% of an alkali metal hydroxide and the balance being at least one strongly oxidizing agent, immersing the article in said solution, closing the vessel to the atmosphere, heating the solution to a temperature of 285 to 308 F. for a period of time suflicient to oxidize the surface of the article, and thereafter removing the article from the treatingsolution.

3. A method of producing black oxide coatings on steel articles, comprising the steps of introducing a treating solution into a vessel, said treating solution comprising a salt composition dissolved in water with the Water being present in the ratio of 1 part water to .77 to 1.31 parts of said salt composition, said salt composition comprising from 50 to parts by weight of an alkali metal hydroxide, 3 to 20 parts by weight of an alkali metal nitrite and 1 to 50 parts by weight of an alkali metal nitrate, immersing the steel article in the solution, hermetically sealing the vessel, heating the solution to a temperature of 285 to 308 F. While maintaining the hermetically sealed condition in said vessel for a period of time sufiicient to oxidize the surface of the steel, and thereafter removing the steel article from the solution.

4. A method of producing black oxide coatings on steel articles, comprising the steps of introducing a treating solution into a vessel, said treating solution comprising a salt composition dissolved in water with the water being present in the ratio of 1 part water to .77 to 1.31 parts of said salt composition, said salt composition comprising from 50 to 90 parts by Weight of sodium hydroxide, 3 to 20 parts by Weight of sodium nitrite and l to 50 parts by weight of sodium nitrate, immersing the steel article in the solution, hermetically sealing the vessel, applying heat to the vessel to heat the solution to a temperature of 285 to 308 F. while maintaining the hermetically sealed con dition in said vessel for a period of 10 to 25 minutes, and thereafter removing the article from the treating solution.

5. A method of producing black oxide coatings on steel articles, comprising the steps of introducing a treating solution into a vessel, said treating solution comprising a salt composition dissolved in water with the water being present in the ratio of 1 part water to .77 to 1.31 parts of said salt composition, said salt composition comprising from 50 to 90 parts by weight of an alkali metal hydroxide, 3 to 20 parts by weight of an alkali metal nitrite and 1 to 50 parts by weight of an alkali metal nitrate, immersing the steel article in the solution, hermetically sealing the vessel, heating the solution to a temperature of 285 to 308 F., applying a pressure in the range of 52.7 to 75.4 p.s.i.a. to the solution, maintaining the solution under said temperature and pressure for a period of 10 to 25 minutes, and thereafter removing the steel article from the solution.

References Cited by the Examiner UNITED STATES PATENTS 2,077,450 4/ 1937 Weisberg et a1 l486.l4 2,192,280 3/1940 Walen et a1. 1486.14 2,355,007 8/1944 Mitchell l486.l4 X 2,817,610 12/1957 Newcll et a1 1486.l4

FOREIGN PATENTS 535,614 4/1941 Great Britain.

ALFRED L. LEAVITT, Primary Examiner. RALPH S. KENDALL, Examiner. 

1. A METHOD OF PRODUCING BLACK OXIDE COATING ON FERROUS ARTICLES, COMPRISING THE STEPS OF INTRODUCING A TREATING SOLUTION INTO A VESSEL, SAID TREATING SOLUTION COMPRISING A SALT COMPOSITION DISSOLVED IN WATER WITH THE WATER BEING PRESENT IN A RATIO OF 1 PART WATER TO .77 TO 1.31 PART OF SAID SALT COMPOSITION AND SAID SALT COMPOSITION COMPRISING FROM 50 TO 90% BY WEIGHT OF AN ALKALI METAL HYDROXIDE AND THE BALANCE BEING AT LEAST ONE STRONGLY OXIDIZING AGENT, IMMERSING THE FERROUS ARTICLE IN THE SOLUTION, HERMETICALLY SEALING THE VESSEL, HEATING THE SLUTION TO A TEMPERATURE OF 285* TO 308*F. WHILE MAINTAINING THE HERMETICALLY SEALED CONDITION IN SAID VESSEL FOR A PERIOD OF 10 TO 25 MINUTES, AND THEREAFTER REMOVING THE ARTICLE FROM THE TREATING SOLUTION. 